Parathyroid hormone-related protein (PTHrP) was originally discovered as the cause of humoral hypercalcemia of malignancy. In recent years, studies using genetically altered mice have demonstrated that PTHrP has important functions during fetal bone development. We have recently used transgenic mice with chondrocyte-targeted PTHrP expression to rescue the PTHrP-knockout mouse from its neonatal death. This has revealed several previously unappreciated extraskeletal developmental consequences of PTHrP gene disruption, including a complete lack of mammary glands due to a disruption of embryonic morphogenesis. There are two principal defects, a failure of the expected androgen-mediated destruction of the mammary buds in males, and a failure of the mammary epithelial cells to initiate branching ductal morphogenesis in females. In fact, the epithelial cells die and are completely absent by birth. This appears to be the result of a lack of PTHrP-signaling to the mammary mesenchyme because PTHrP is expressed in mammary epithelial cells and the PTH/PTHrP receptor is expressed in the mesenchyme. These results illustrate a central theme of PTHrP's developmental effects, its participation in epithelial-mesenchymal interactions during the formation of epithelial organs. Our central hypothesis is that PTHrP is an epithelial signal that is required to condition the mesenchyme in order for it to support the morphogenesis and survival of the epithelial cells. To explore this hypothesis, we propose three specific aims. Aim 1 seeks to characterize the nature of the mammary epithelial cell death and seeks to test if PTHrP is also necessary for epithelial cell survival and/or morphogenesis after embryogenesis is complete. Aim 2 seeks to examine the mechanisms underlying the failure of sexual dimorphism in the knockout mammary buds and to test whether PTHrP is necessary for the functional differentiation of the mammary mesenchyme. Aim 3 seeks to characterize changes in gene expression in mammary stromal cells in response to PTHrP, in order to identify mesenchymally-derived factors acting downstream of PTHrP to promote branching morphogenesis of the epithelium. These studies will offer important insight into the mechanisms underlying PTHrP's effects during development, both in the mammary gland as well as in the skeleton and its many other sites of expression in the embryo.